1. Field of the Invention
The present invention relates to an ink composition that has excellent dispersivity of a colorant, excels in color developing ability, can be cured by irradiation with active radiation, and can form high-quality images, this ink composition being especially suitable for ink-jet recording applications. The invention also relates to an image recording method and image recording matter using the ink composition.
2. Description of the Related Art
An electrophotographic method, a thermal transfer method of sublimation and melting type, and an ink-jet method are known as image recording methods for forming images on a recording medium such as paper based on image data signals. Among them, the ink-jet method excels as an image recording method because it can be realized with an inexpensive apparatus and forms images directly on the recording medium by discharging the ink only on the required image portion, thereby making it possible to use the ink efficiently, has a low running cost, and has a low noise level. With the ink-jet method, printing can be performed not only on regular paper, but also on recording media that do not absorb water, such as plastic sheets and metal sheets. However, how to increase speed of printing and quality of printed images is the problem associated with the ink-jet method. In addition, a specific feature of this method is that the time required for drying and curing the droplets after printing greatly affects the sharpness of images.
An example of the ink-jet systems is a recording system using curable ink compositions for ink-jet recording that can be cured by irradiation with active radiation. With such a system, sharp images can be formed by irradiating the ink droplets with radiation and curing them immediately after printing.
In order to form highly accurate images with excellent color developing ability, the curable ink compositions are required to have high pigment dispersivity and stability. Generally, the size of pigment particles has to be decreased to impart the ink composition with a bright color tone and high tinting strength. In particular, in ink compositions for ink-jet recording, because the discharged ink droplets greatly affect the sharpness of images, the discharge ink droplets also have to be small and very fine pigment particles that are less in size than the thickness of the cured ink film formed by the ink composition have to be used. Where the size of pigment particles is thus further reduced to obtain a high tinting strength, the pigment becomes difficult to disperse, pigment aggregates are formed, and the viscosity of ink composition increases if a pigment dispersant is added. The appearance of pigment aggregates and the increase in the ink composition viscosity produce an adverse effect on the ink discharge ability and greatly degrade the performance of ink composition. Yet another problem is that because the ink composition used for the ink-jet method is accommodated in a cartridge, heated during discharge, and maintained at a reduced temperature when it is not discharged or during storage, the ink is subjected to cyclic temperature variations of heating and cooling, such temperature variations also adversely affect the pigment dispersivity, and the dispersivity of pigment decreases with the passage of time, easily causing thickening or aggregation in the ink.
For this reason, there is a need for an ink composition that has sufficient flowability, can disperse a finely powdered pigment with excellent stability, and excels in stability of pigment dispersion with time, and a variety of dispersants for obtaining a stable pigment dispersion have heretofore been suggested.
For example, an ink composition using a pigment derivative as a dispersant to increase the affinity for the pigment (see Japanese Patent Application Laid-Open (JP-A) Nos. 2003-119414 and 2004-18656), an ink composition using a polymer having a basic group as a dispersant for a special pigment such as phthalocyanine- and quinacridone-based pigments (see JP-A No. 2003-321628), and an ink composition using no organic solvent and containing a dispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and an organic solvent containing a special monomer that dissolved the dispersant (see JP-A No. 2004-131589) have been suggested.
However, in the case of these ink compositions, although the pigment dispersion stability has been increased due to the functions of the dispersants with respect to those of the conventional ink compositions, the decrease in the size of the pigments used is insufficient and there is still room for improvement in terms of increasing the dispersivity of fine pigment particles. One more problem, it that dispersion stability after long-term storage or cyclic temperature variations is insufficient.